Heatwave and wildfires worsened Colorado flooding

A TRULY ferocious and exceptional event. That's how Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colorado, describes the storm that pummelled his state last week. "This was a once-in-1000-year rainfall."

The torrential rains and subsequent floods have so far killed eight people, displaced 11,750 and damaged close to 18,000 homes. The city of Boulder received a year's rainfall in less than a week, says Daniel Leszcynski at the National Oceanic and Atmospheric Administration. That huge volume was due in part to a lingering heatwave that for months blocked tropical moisture from the Gulf of Mexico from reaching the Rockies, he says. When that heatwave began to move east last week, weak winds allowed the growing storm system to sit above the Colorado peaks for days.

Once that deluge hit the ground, more trouble awaited. Recurring wildfires near Boulder and Fort Collins in recent years had cleared the land of vegetation that would normally absorb rainwater in these areas.

Though natural disasters are difficult to attribute to climate change, Trenberth says that the 1 °C rise in ocean temperature since the 1970s accounts for 5 per cent more moisture in today's atmosphere. That's enough to invigorate already powerful storms such as last week's, he says.

Correction:When this article was first published on 18 September 2013, it said that nearly 18,000 homes had been destroyed rather than damaged. This has now been corrected

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